1. Field of the Invention
This invention is related generally to materials and methods for the fabrication of radiation curable composites and coatings, and more particularly to materials and methods for forming radiation curable polymer network coatings and fiber reinforced pre-pregs and structural composites.
2. Background of the Invention
The use of radiation to cure coatings or composites is advantageous because environmentally hazardous solvents are not released during the curing process. In addition, the curing takes place very quickly, and at low temperatures compared to conventional thermal curing. The lower temperatures made possible with radiation curing reduce residual mechanical stresses that accompany conventional thermal curing.
Several processing methods are known for curing composites with radiation, and they are typically divided into two general categories: liquid state radiation curing, which requires the use of a mold, and solid-state radiation curing, which allows the curing of free-standing objects.
Liquid state radiation curing usually involves the injection of a resin into a mold containing reinforcement fibers. Electron beams penetrate into the mold to crosslink the liquid resin matrix. The mold determines the composite's final shape after it is crosslinked to a solid.
Solid state radiation curing involves first producing a self-supporting and easily handled solid object known in the art as a "green-body", followed by radiation curing. The intermediate stage of producing green bodies is known in the art as the "B-stage". Curing while the resin matrix is in a solid state is also known as "free-standing" curing, since a mold is not required during the radiation curing process. The mechanical and thermal properties of a green-body object are improved by subsequent radiation crosslinking.
There are a number of processing and cost advantages to free-standing curing. U.S. Pat. No. 5,173,142, issued on Dec. 22, 1992 describes how free-standing curing can be used to assemble a hollow section assembly. The cost advantages of using free-standing curing instead of conventional steel stamping at moderate production quantities are described in the technical article by D. L. Goodman, D. L. Birx, G. R. Palmese and A. Chen, entitled "Composite Curing with High Energy Electron Beams," in the Journal of the Society for the Advancement of Material and Process Engineering Vol. 41, 207 (1996).
A generic problem in fabricating large structural composite parts is the difficulty of part fit-up. In applications involving the assembly of multiple parts, such as in automotive or aircraft applications, good part fit-up (which refers to how well parts fit together) is necessary to allow final assembly, as well as for aerodynamic and aesthetic reasons. Unlike ductile metals, where the final shape can be modified by mechanical means at the final stages of assembly, rigid polymer matrix composites must achieve part fit-up by maintaining accurate fabrication tolerances.
In conventional composite processing (such as autoclave curing of aircraft composites), part fit-up problems are primarily caused by residual thermal stresses due to thermal expansion mismatch between fiber reinforcement and resin matrix upon cooling.
A major advantage of low-temperature curing processes such as radiation curing is a reduction in residual thermal stresses associated with conventional thermal curing methods. However, radiation curing has an analogous problem: resin shrinkage during curing. In composites, resin shrinkage leads to internal stresses which can cause composite warpage and poor part fit-up. Shrinkage also leads to higher process engineering costs, since molds must be oversized or oddly-shaped to compensate for shrinkage and warpage. In coating and pre-peg applications, shrinkage can contribute to poor adhesion, especially to metal substrates.
Many conventional resin systems used in radiation curing shrink significantly when cured. Volumetric shrinkage values for common acrylic monomers arc given in a 1964 Rohm and Haas Company (Washington Square, Pa.) technical report entitled "Volume Shrinkage During Polymerization of Acrylic Monomers," and range from 11-30%. Linear shrinkage of acrylated oligomers are listed in a 1996 UCB Radcure Product Specification entitled "UV/EB Curable Oligomers and Shrinkage Behavior," available from UCB Chemicals Corp., Radcure Business Unit, 2000 Lake Park Dr., Smyrna, Ga., and correspond to volumetric shrinkages of 7-25%. A typical resin formulation, incorporating such oligomers and monomers, will have a volumetric shrinkage of 9-15%.
As described in Chapters 1 and 9 of the book "Radiation Curing, Science and Technology," edited by S. P. Pappas (Plenum Press, NY, 1992), one possible approach to reducing shrinkage of radiation-curable resins is the use of cationic polymerization via cationic initiators possessing highly non-nucleoplilic anions (PF--, AsF-- or SbF--). This is an alternative to the more common radiation-curable resin systems of the free-radical type, which possess carbon-to-carbon double bonds that cross-link upon exposure to radiation.
Radiation-induced cationic polymerization has shown lower shrinkage (typically about 6%) than free-radical systems. Cationic catalysts are, however, easily poisoned by nucleophilic resin components, contaminants or fiber sizings, making radiation curin via cationic mechanism difficult. This is especially true for free-standing curing, where the presence of weakly nucleophilic urethane groups or of highly nucleophilic resin "B" components such as amines are incompatible with the use of cationic catalysts.
There is thus needed a method for formulating radiation-curable resins with low shrinkage for use in composite and coating applications, especially where free-standing curing is desired. There is also need a method which eliminates or overcomes the poisoning problem associated with cationic polymerization.
Accordingly, it is an object of the invention to provide a radiation curable polymer network.
Another object of the invention is to provide a radiation cured polymer network that has a reduced shrinkage and warpage.
Another object of the invention is to provide a method for forming radiation curable coatings, composites and pre-pregs made with the radiation curable polymer network of the present invention.